A. Field of the Invention
This invention relates to object inspection apparatus, and more particularly, to an automatic reticle inspection system which utilizes an advanced alignment technique capable of detecting defects in VLSI reticles by comparison to a stored database.
B. Description of the Prior Art
Automatic object inspection systems that use an object comparison fault detection technique have been described in the literature for some time and have been commercially available for a number of years. The technique generally involves scanning the object to be inspected with an electro-optical or other pick-up device and developing an image-like (bit-map) electronic representation of the object so a comparison to a known good electronic representation (bit-map or database) can be made. The known good electronic representation can be obtained by simultaneously scanning a known good object in synchronism with the scanning of the object of unknown quality to develop the bit-map, or by using a previously stored electronic representation (either bit-map or database) of a known good object.
In both of the above comparison techniques, the fault detection method consists of locating differences between the compared objects. To accurately determine faults, the system must register or align the two objects, or electronic representations of the objects, prior to comparison, so that differences caused by actual faults, as opposed to differences merely caused by misregistration, can be detected. Accordingly, the quality of the registration in these systems limits the overall performance or sensitivity of the system to actual faults, that is, the system cannot be so sensitive that it detects misregistration errors. Similarly, the ability of the system to detect very small faults is also determined by the spatial resolution of the pick-up method. In other words, to detect very small faults, the spatial resolution of the system must be very high.
In practice, systems with high spatial resolution, which are used to inspect objects occupying a large surface area, must process data at high speeds in order to complete the inspection in a reasonable and practical amount of time because of the large volume of data. Furthermore, such high spatial resolution systems are more likely to be affected by misregistration errors caused by the mechanical nature of the pick-up device; small differences in spatial distortion, vibration, rotation, displacement, thermal effects, and similar problems can result in significant misregistration errors when the two objects are compared. Such misregistration errors must be controlled or compensated for in order for a system to provide high performance.
In the integrated circuit inspection system described by Micka U.S. Pat. No. 3,909,602, an optical scanner with a 2.5 micron spot size is used to scan a typical chip, having 0.2 inch by 0.2 inch surface dimensions. The unknown device is scanned and the resulting signal is compared with a known good signal. The known good signal is either generated by simultaneously scanning a known good device, or by replaying a stored representation which has been previously obtained by scanning a known good device. Creating a stored representation such as is taught in Micka, would typically require 4.times.10.sup.6 data words and would typically require reading at a rate of less than 600 KHZ. Micka suggests that this apparatus may be used to inspect some devices or photomasks.
Micka teaches a method for finely adjusting the physical position of the device or devices being scanned for the purpose of static registration. Micka points out that a point-by-point comparison would not be satisfactory because of misalignments that might occur. Thus, Micka teaches a comparing system which uses many spots in a neighborhood, with a cross-correlation technique, to provide higher sensitivity than would be obtainable with a simple point-by-point method. One disadvantage to this system is that it depends on static positioning for registration and provides no means by which to dynamically compensate for registration errors, other than providing a detection method which is somewhat resistant to registration errors.
Another disadvantage to this system is that reducing the scanning spot size (such as to 0.5 microns) or scanning objects with larger surfaces (such as 0.5.times.0.5 inches) when inspecting VLSI devices, photomasks or reticles can result in a stored data volume requirement greater than 6.times.10.sup.8 words, and require a data rate greater than 6 MH.sub.z, just to complete the inspection in a reasonable amount of time. Achieving this data volume and data rate is not practical with presently available storage media. Hence, the technique described by Micka is not practical for the inspection of VLSI devices. Even common compression methods such as run-length encoding or DPCM will produce only a 2.times. to 3.times. compression in this type of application, which still results in excessive data volume.
An additional disadvantage of the apparatus taught by Micka is that the known good device must first be inspected by other means and certified to be good before it can be used as a master. A human operator can inspect larger devices by means of a microscope, but to inspect VLSI devices, with sides as small as 0.5 inches, by such means, would require microscope magnification as high as 1000.times.. At 1000.times. magnification, each field-of-view would be extremely small, and as many 62,000 fields-of-view would be required to examine the whole device. The use of human operators for such inspections has proven to produce significant operator errors and fatigue.
An alternate approach is to electrically test the device, with certification dependent on the device passing the test. Not all faulty devices will be detected by electrical testing because some process errors do not produce immediate failures. Faulty certification of the master can result in even greater problems when applied to certifying VLSI devices. In order to electrically test devices, the devices must first be fabricated, and then electrically tested. A fabricated device that passes a first time-test may actually be good, but the device could also have a hidden defect that would not be exposed until later. If the device fails, the fault may be due to a photomask or reticle fault, or some other process fault. When a device fails, there may be no way to determine which is the actual cause. This method of certification is poor at best and results in a considerable waste of time and money. Thus, the inability to accurately certify a VLSI device prior to usage of the Micka apparatus proves to be a significant disadvantage.
A solution to this problem was offered by Kryger, U.S. Pat. No. 4,218,142, wherein an apparatus is described in very general terms which compares an unknown photomask to an electronic representation stored in a high-speed memory device. However, in Kryger, the memory device only stores a small portion of the total electronic representation and must be successively reloaded by the computer with the next section after each applicable portion of photomask has been inspected. A disadvantage to this approach is that a significant amount of time must be spent reloading the memory device from the relatively slow computer. The time disadvantage would be particularly acute when inspecting single-die or multi-die reticles where few repetitions of the die pattern exist, and where large die and small pixel sizes must be used, resulting in a very large data base size, such as 6.times.10.sup.8 words. Thus the Kryger technique apparently provides a slow and inefficient process for inspecting VSLI photomasks and reticles.
The object inspection apparatus described by Lloyd et al, U.S. Pat. No. 3,916,439, uses a TV camera, as the optical pickup device, to compare the TV image of the unknown device with a previously stored image from a known good device. The comparison technique uses the point-by-point method, and the synchronization technique uses line-by-line timing of the TV camera to obtain corresponding data from the storage device. One disadvantage of this type of system is that the synchronization method, comparison method and storage method are not practical for high resolution large area applications like VLSI photomasks and reticles, for many of the same reasons discussed above.
The object inspection apparatus described by Kurtz et al, U.S. Pat. No. 4,240,750, uses a laser scanner as the optical pick-up method for the purpose of inspecting printed circuit parameters. In one embodiment, the apparatus measures the angular position of a lead wire and then compares that position with a desired position. This technique is not applicable to the type of inspection discussed herein. A second embodiment is similar to the type of object inspection techniques described by Applicant. In this embodiment, the printed circuit board to be inspected is scanned with a rectangular raster type sweep with a spot size of about 1.5 mils (37 microns) and a typical area of 3.times.3 inches, resulting in about 4.times.10.sup.6 data points.
The known good board is first scanned and the resulting electronic representation is stored in a memory. An unknown board is then scanned while simultaneously comparing the electronic representation with the stored representation. Faults are located by performing a point-by-point comparison of the two electronic representations. The synchronization method applied merely consists of the scanner position and memory being addressed from the same counter. Thus, registration is statically accomplished by insuring that the unknown board is positioned in the same position as the known good board, when it was scanned. The disadvantages to this system, when extended to the inspection of VLSI photomasks or reticles, are the same as those mentioned in the previous paragraphs regarding the Micka apparatus.
The object inspection apparatus described by Levy et al, U.S. Pat. Nos. 4,247,203 and 4,347,001, is used for the inspection of photomasks used in the manufacture of semi-conductor devices. These apparatus locate faults in the photomask by simultaneously comparing adjacent die on the photomask and locating differences. Because a known good die is not used in this type of inspection, only random faults can be located, thereby leaving repeating faults still present. Furthermore, Levy states that no storage method is included, due to the large data volume that would be required.
The apparatus described in Levy is designed to be capable of locating 1.25 micron faults. Levy disclose that the mechanical tolerances of such a high sensitivity inspection system, imperfections in the photomask, and operator misalignment of the photomask to be tested can cause the scanned images to be slightly misaligned. This misalignment changes with time during the inspection and with the position of the mask. Unless the misalignment can be compensated for or predicted, defects smaller than the misalignment cannot be detected.
In order to correct any misalignment, Levy discloses an alignment method consisting of a high-speed memory, which can be used to delay the electronic representations from the two die being compared relative to each other to effect a dynamic change in the registration, and a dynamic alignment error detection method, to determine when a registration change needs to be made. Since the objects to be inspected are located on the same plate, many of the misregistration error sources are common to the detection system, and affect both die, such as stage vibration, stage velocity changes, plate rotation, etc. There are also additional error sources which are not common to both die, such as relative vibration of the two optical pick-up units (objectives) or die run-out present on the plate.
The alignment method taught by Levy uses patterns within the die for error information and is capable of dynamically detecting and correcting .+-.1 pixel of misalignment over a total range of .+-.8 pixels. However, the method is limited to only slow moving misalignments. In addition, as is stated in the disclosure, the alignment method cannot maintain proper alignment when there is no pattern information on the plate for large distances, or if there are sudden changes in registration as might be encountered with a stepping error on a particular die. The overall sensitivity of the apparatus is preserved by the inclusion of a detection method which can, to some extent, tolerate the above mentioned situations, as might be encountered on photomasks.
One disadvantage to the system taught by Levy is that there is no means for locating repeating defects or for inspecting single die reticles since there is nothing to compare against. With the exception of Kryger, in all previously mentioned prior art, a known good stored representation must already exist for there to be anything for the system to use for comparison. Combining any of the stored data methods taught by Micka, Kurtz, or Lloyd, and even that taught by Kryger, to the apparatus taught by Levy would require substantial redesign of the combined systems which is neither suggested nor taught by any of the references. Such a combination would not be able to accommodate the high data volume (as described earlier) and the even higher data rate requirements (as high as 20 MHz) of a system analogous to the present inventions. Furthermore, even if it were possible to produce the stored representation at the required data rate, the alignment method taught by Levy would be far too inadequate to maintain proper alignment between the stored representation and the object being scanned.
The alignment system taught by Levy cannot be effectively extended to make a comparison of a photomask or reticle with a stored representation. Such a use would not be possible because the resulting misalignment errors that would exist would be too great in number, and all of the errors that the two die formerly had in common, such as stage velocity errors, stage vibration, optical pick-up vibration, rotation, and thermal expansion, would only be applicable to the unknown die. Thus, the alignment system would not only have insufficient error detection capability, but would also have insufficient responsiveness.
In addition, since 5.times. and 10.times. reticles, or the like, have significantly larger patterns than a 1.times. photomask, large areas of the plate may contain no pattern. When there is no pattern on the plate, the alignment system cannot track the alignment. Thus, when the pattern is again encountered, the alignment error may be beyond the range of the error detection method and the alignment system, and may not have sufficient responsiveness to prevent detection of any faults solely due to misregistration. Furthermore, an extension of the described technique to .+-.8 pixels would require up to 50 times more computation, thereby making the system too complicated and slow to be practical. Thus, a reticle inspection system using the above described alignment system would have poor sensitivity, due to the inability of the alignment system to maintain proper alignment.